Communication systems are known to support wireless and wire-lined communications between wireless and/or wire-lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera, communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or multiple channels (e.g., one or more of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel or channels. For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel, or channels. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the receiver receives RF signals, demodulates the RF carrier frequency from the RF signals to produce baseband signals, and demodulates the baseband signals in accordance with a particular wireless communication standard to recapture the transmitted data. The receiver is coupled to an antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies them. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signals into the baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out-of-band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
As is also known, the transmitter converts data into RF signals by modulating the data to produce baseband signals and mixing the baseband signals with an RF carrier to produce RF signals. The transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts the raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce the RF signals. The power amplifier amplifies the RF signals prior to transmission via the antenna.
The local oscillations used in both the transmitter and in the receiver may be produced by the same or different local oscillation generators. Such a local oscillator may be in the form a phase locked loop (PLL), where the output frequency of the PLL is used as the local oscillation. Local oscillators used in direct conversion or very low intermediate frequency (VLIF) transmitters and receivers include a PLL that produces an output frequency of approximately ⅔rd of the desired local oscillation. To obtain the desired local oscillation, the output frequency of the PLL is divided by 2, creating a frequency of approximately ⅓rd the desired local oscillation and then added to the original ⅔rd of the desired local oscillation. For example, as shown in FIG. 1, the PLL may produce an output oscillation of 1600 MHz, which divided by two equals 800 MHz, and, when mixed together, the resulting local oscillation of 2400 MHz is obtained.
As is also shown in FIG. 1, the local oscillation can be fed into a frequency shift keying (FSK) modulator (an I-Q mixer) in which an FSK baseband signal is directly mixed with the local oscillation signal to form an FSK modulated signal. The FSK modulated signal is an RF signal that can then be fed to, for example, a transmitter power amplifier.
A shortcoming with any I-Q mixer is keeping proper 90 degree phase-shifts on all ports of the mixer to prevent feed-through of the image frequency, which is the opposite signed portion of the baseband data signal riding on the local oscillation from that portion of the baseband data signal being transmitted. In an on-channel FSK modulator such as described above, the image frequency falls in-band (e.g., local oscillation frequency+/− baseband signal frequency), so excellent balance must be maintained across frequency, temperature, and integrated circuit process variation in order to attain an acceptable output signal from the FSK modulator. As frequencies get higher, the I-Q balance is difficult to achieve, resulting in in-band image frequency leakage that adversely affects the signal-to-noise ratio (SNR) of the modulated FSK signal.
Further, prior to transmission of the modulated FSK signal, it is passed through a power amplifier (PA) that amplifies the signal by, for example, 20-30 dB. Applying such gain to a high frequency signal can result in feedback of the amplified modulated signal to the FSK modulator, which corrupts the original modulated signal. Such corruption results because the feedback signal is phase-shifted from the original modulated signal, which disrupts the I-Q balance of the modulated signal, resulting in increased image frequency feed-through or other adverse effects, such as variance in the signal amplitude. In an on-chip radio system there are typically several feedback paths that are layout dependent. These feedback paths can cause varying amounts of phase and amplitude feedback into the FSK modulator. As a result, radio systems incorporating a prior art local oscillation generation and high frequency modulation scheme will suffer from in-band image frequency leakage and feedback of the amplified FSK modulated signal into the FSK modulator, resulting in a high signal-to-noise ratio and degraded RF signals.
Therefore, a need exists for an FSK modulator and local oscillation generator that reduce and/or eliminate in-band image frequency leakage and feedback problems associated with the prior art.